It is often desired to supply fluid form a high-pressure source to a consumer conduit at a given reduced pressure while maintaining that pressure for all flows required by the consumer. One example is the supply of gas from a storage vessel of liquified gas such as oxygen or nitrogen. Liquified gas form the storage vessel is pumped into a vaporizer comprising heat exchanging tubes having fins exposed to natural convection in the atmosphere. In an alternate method, the pressure of evaporated liquified gas is allowed to build up in the vessel, thereby pressurizing the vessel and the liquid delivered form the vessel to the vaporizer. The pressure regulation task is made particularly difficult in that as the liquid cryogen boils in the vaporizer, the boiling front location shifts unstably back and forth causing sizable swings in the pressure. Swings of .+-.20 psi are typical from such vaporizer surging. From the vaporizer, the gas is regulated to the desired pressure and delivered into the consumer conduit for supply to the consumer's process. The process may consume gas at varying rates and include periods of nonconsumption.
Such an installation is expected to function unattended in all weather conditions. Automatic protection against unusual circumstances therefore is required. The delivery of gas at too cold a temperature, or the delivery of unevaporated liquid is to be prevented to avoid thermal shock, embrittlement and upset of the consumer's facilities. Thus a device is desirable to interrupt the supply of fluid to the consumer conduit if the fluid emerging from the vaporizer falls below a given temperature, for example, -40.degree. F.
Available for the desired service are a host of capable electrical and pneumatic controllers employing feedback loops and offering sophisticated actions such as proportional, reset and rate response to deviations from a set point. These controllers, however, are complex, costly and require maintenance of a high degree of skill. Electrical controls of this type also require protection against extremes of cold, hot and inclement weather.
Simple, self-contained regulators are also available which when operated over a wide range of flow provide only a rough degree of pressure regulation. These designs typically balance a loading force against the force of the outlet pressure of the regulator acting over the area of a diaphragm. The diaphragm deflects in the direction of the larger of these forces. A valve is linked to the diaphragm so that when the loading force is larger, the valve will open, and when the force of outlet pressure acting over the diaphragm is larger, the valve will close. The loading force must be held constant in order to hold downstream pressure steady.
A spring is commonly used to supply the loading force on the diaphragm. Usually the force the spring exerts against the diaphragm can be varied with an adjusting screw. As the diaphragm deflects, the length of the spring changes and thereby the loading force. This effect generates a serious control error that becomes greater as flow varies from zero to maximum.
Another major source of control error in prior art regulators is due to the force of inlet pressure acting on the valve plug. This force is transmitted through the linkage to the diaphragm and acts against the loading force. The set point provided by the spring force is effectively reduced as inlet pressure rises, and increased as inlet pressure falls. In order to minimize this effect the valve orifice is typically made small resulting in low capacity compared to the size of the valve.
The force to deflect the diaphragm represents yet another prior art regulator control error. This force against opposes the loading force reducing the set point as the valve opens.
Still another prior art control error is introduced by the amount of force required to seal the valve over and above the force exerted by inlet pressure. This phenomenon called lockup, is determined by the stiffness and surface finish of the mating sealing surfaces.
Commonly, the performance of simple regulators is improved by using a pressure loaded configuration in which the diaphragm is loaded with fluid pressure rather than the spring described earlier. Such practice addresses the control error caused by the spring, but not the additional factors discussed above. Pressure loading is accomplished by employing a pressure tight chamber called the dome whose inner surfaces include the diaphragm. The dome is commonly pressurized using a spring loaded regulator called a loading regulator to reduce pressure from a loading pressure source to the required pressure. The loading pressure source may be either the inlet fluid which is being regulated or a separate fluid source such as instrument air.
The volume of the dome changes as the diaphragm deflects to open and close the valve. In order to hold the dome pressure constant, loading fluid must be added when the diaphragm deflects to open the valve, and loading fluid must be released when the diaphragm deflects to close the valve. Since the loading regulator can only add gas to the dome, a variation in loading pressure results. Some improvement in holding loading pressure constant can be achieved by specifying a loading regulator with a self relieving feature. Such a regulator vents loading fluid when the downstream pressure exceeds the set value by an increment dependent on its specific design. The variation in loading pressure is thereby limited to this increment.
An object of the present invention is to provide a method and system for controlling a process fluid flow over a wide flow range so as to maintain a prescribed downstream pressure within moderately accurate limits. Another object is to provide an apparatus to reduce or terminate the process fluid flow should the process fluid temperature drop below a prescribed temperature.
Features of this invention are that its working items are self-operated, simple and rugged. No additional power source or control fluid source is required.
Advantages of this invention are low initial cost and low maintenance requirements of moderate skill level.
Other objects, features and advantages are pointed out and become apparent in the subsequent description of the invention.